1. Field of the Invention
The invention relates to the forming of felts, the fibres of which are disposed in random directions. More particularly, the invention relates to felts formed from mineral fibres which are designated by the generic names of glass wool, rock wool, etc.
Traditionally, mineral fibre felts are constituted continuously by depositing on a conveyor the fibres which are carried by currents of gas. The conveyor retains the fibres and allows the gases to pass through it.
Before they are deposited on the conveyor, the fibres are coated with a resinous composition intended to bind the fibres to one another, so imparting their cohesion to the felt which is thus constituted. The resinous composition applied in liquid form is cross-linked by heat treatment carried out on the felt which has been previously brought to the desired conditions of thickness and volumetric mass.
The conventional methods of felt formation result in products, the properties of which do not entirely satisfy all the demands imposed on them by certain particular applications. Over and above the insulating properties which are quite generally required, it is thus sometimes necessary for the products used to have very specific mechanical qualities. That is, for example, the case with products which support masonry elements and which must therefore withstand high levels of compression, such as elements which are used for the insulation of flat roofs which are accessible to traffic. It is also the case with products used in outdoor insulation and which must in particular be able to withstand tearing stresses.
In order to obtain products which have these specific qualities and also others which we will examine hereinafter, it is necessary to modify the conventional felt forming processes.
Felt formation by the deposition of fibres on the receiving conveyor or on a similar member leads to tangling which is not homogeneous in all directions. Experimentally, it is found that the fibres have a marked tendency to assume positions parallel with the receiving surface. This tendency is all the more marked the longer the fibres are.
This structure of felts is favourable to their insulating properties and also to their resistance to traction in the longitudinal direction. Such a structure is consequently advantageous for many uses. However, it will be appreciated that such a structure may not be the most suitable if, for example, the product is required to withstand compression or tearing in the direction of its width.
In order to improve resistance of these felts to compression, one solution lies in increasing their volumetric mass by increasing the number of fibres per unit of surface area on the receiving member where the felt is constituted. Apart from the fact that the mass of fibres per unit of surface area which can be deposited is limited, the accumulation of fibres on the receiving member rapidly opposes the passage of gases and therefore continuance of felt formation under good conditions, which makes it difficult if not impossible to improve other properties such as tearing resistance.
Another solution previously proposed resides in proceeding in such a way that the direction of the fibres is no longer in the plane of the felt but in a plane at a right-angle thereto. This arrangement is achieved, for example, by forming creases in the felt. These creases are in particular obtained either by disposing the felt in successive layers of greater or less length, extending in the direction of the desired final thickness, or by compressing the felt longitudinally. Under the effect of compression under the conditions envisaged, the felt forms undulations. Heat treatment of the binder composition which is carried out subsequently imparts a permanent character to this folded structure.
The direction of the fibres oriented in the direction of the thickness of the felt thus formed makes it possible substantially to improve the compression and tearing resistance. This structure is, however, disadvantageous with regard to resistance to longitudinal traction--the felt having a tendency to become unfolded--or to flexion.
The disposition of the fibres in the thickness may result also from assembling together strips of felt, the width of which corresponds to the thickness of felt desired, each strip being disposed in such a way that the fibres are in planes perpendicular to the faces of the constituted felt. The strips are maintained against one another by means of a coating or a film covering one or both faces of the felt. Possibly, the strips may also be glued directly to one another by their contacting surfaces.
The felts produced by this relatively complex technique, referred to as "layered cloths", are used mainly for insulating pipes. For this application, the flexion or even rolling capacity of the product obtained, instead of being a disadvantage, is particularly desirable.
An aim of the present invention is to provide felts in which the mechanical properties, particularly the resistance to compression and tearing in the direction of the thickness of the product, are improved without incurring any of the disadvantages previously encountered and consequently without any formation of creases, and without assembling together strips of felt.
Another object of the invention is likewise to provide felts having improved mechanical properties, and the insulation properties of which are still satisfactory.
Another object of the invention is to provide felts which exhibit the previously mentioned properties, while having the lowest possible volumetric mass.